Modulation of Therapeutic Response Summary In the interest of improving cancer treatment, considerable attention has been placed on the modification of radiation damage. The interaction of a variety of chemotherapy and/or molecularly targeted agents with radiation is under study to determine if tumors can be made more sensitive or normal tissues more resistant to radiation treatment. The central aim is to identify approaches that will result in a net therapeutic gain, thus improving cancer treatment with radiation. One goal of the project is to define and better understand those aspects of tumor physiology, including cellular and molecular processes and the influence of the tumor microenvironment on treatment response. Two independent studies have been completed showing that radiation-induced gene expression profiles differ significantly for cells exposed in vitro versus the same cells growing as a solid tumor in vivo further underscoring the influence of the tumor microenvironment on the radiation response. Further we have shown that multi-fraction radiation delivery results in a more robust induction of genes than single dose radiation treatment. With respect to the tumor microenvironment, it has long been known that tumor cell hypoxia can cause resistance to radiation and chemotherapy treatment. Non-invasive imaging that could identify patients whose tumors are hypoxic would be useful clinically. A putative radioactive hypoxia marker, [64Cu]Cu-ATSM, was evaluated in tumor-bearing mice using dynamic positron emission tomography (PET). While this PET tracer avidly bound to tumor, it was unable to predictably detect changes in varying amounts of hypoxia when oxygenation levels of the tumor were modulated. In contrast, [18F]fluoromisonidazole demonstrated a positive trend with more tumor uptake as oxygen levels were lowered in the tumor. Future attempts of non-invasively assessing tumor hypoxia will be evaluated using electron paramagnetic resonance imaging (instrumentation developed in the Radiation Biology Branch) in conjunction with oxygen sensitive chemical probes. The ability to enhance the response of the tumor to radiation, without enhancing normal tissue within a given treatment field is desirable. We have recently shown that loratadine and guggulsterone enhance tumor cell radiation response in vitro. Preliminary mechanistic studies indicate that loratadine imposes a G2/M block in cell cycle (G2/M phases of the cell cycle are very radiosensitive) and guggulsterone, which appears to interfere with radiation-induced DNA damage repair. Evaluation of both agents in combination with radiation in tumor bearing mice is currently underway. With respect to normal tissue response to radiation, it is widely known that the TGF beta signaling pathway is a major player in radiation-induced late effects (fibrosis). Our previous studies have shown that mice deficient in TGF beta signaling (Smad3 knock-out mice-downstream signaling intermediate in the TGF beta pathway) are resistant to fibrosis when treated with high dose radiation. We have recently shown that fibroblasts isolated from these animals (compared to wild type fibroblasts) when exposed to radiation increase their DNA damage sensing mechanisms and decrease induction of pro-fibrotic genes. These basic studies suggest that targeting the TGF beta pathway with molecularly targeted agents may provide significant protection against radiation-induced fibrosis, an untoward side effect of radiation treatment. Recent mouse normal tissue studies using halofuginone, which targets the TGF beta pathways at several points or a TGF beta type 1 receptor kinase inhibitor have shown marked reduction in radiation-induced soft tissue fibrosis. Current studies are centered in confirming that both agents impact their molecular targets in vivo. The goal of these pre-clinical studies is to gain enough efficacy data to introduce these agents into human clinical trials.